There have been a number of conversations about KTM LC4 exhaust systems and slip-ons, so I did a little research (very little) and came up with a few lesser known brands, at least in the US, to consider besides the factory KTM stuff, the Supertrapp IDS2, Big Gun and the on again off again Pro Circuit T-4.
So far here is a short list of some of the other brands:
Sebring, BSM by ABP, Leovince X-3, SP, Doma, Akrapovic. I'm sure there are others out there, but I haven't found them... yet.
The first link is the main page for Schnider Cycles in Germany, the second for their english power parts pull down menus. The third is for ABP and is in german.
My thoughts on this whole pipe thing is that there are lots of styles, materials, price ranges etc, but I doubt any of them, within a given sound db range, offer any dramatic improvement over one another. So go with exclusivity, features, finish, quality, performance, sound, price... what ever your priorities are.
I personally went with a Supertrapp IDS2 because of the quality, price, USFS spark arrestor, the ability to fit it ( with a minor modification) to my '03 Adventure and the wide range of tunability, and as I am a friend of a Parts Unlimited dealer... easy to get.